The present invention relates generally to audio power amplifiers and more particularly, this invention pertains to audio power amplifiers using feedback signals to control power supply output. It is known in the prior art to use an audio power amplifier powered by a regulated switching power supply with both positive and negative outputs. Due to the nature of an audio power amplifier driving low frequency signals, the positive and negative outputs are loaded sequentially. Due to the nature of a switching power supply, the loaded output""s voltage is reduced, or sags, making less power available to the load. In addition to this problem, at the same time the unloaded output""s voltage is increased, or floats, which can cause an overvoltage condition and failure of components when their ratings are exceeded. A third problem occurs when the dissipation of the power amplifier is too large for continued operation, as may be caused by excessive ambient temperatures, or by excessive loading of the amplifier. In this situation, an unregulated power supply cannot respond to remedy this condition and typical prior art has involved shutting down the amplifier until it is cool enough to run again. The present invention is directed to these problems.
Several United States Patents discuss the designs of amplifiers or power supplies. These include: U.S. Pat. No. 6,097,249 entitled Method and device for improved class BD amplification having single-terminal alternating-rail dual-sampling topology; U.S. Pat. No. 5,932,827 entitled Sustainer for a musical instrument; U.S. Pat. No. 5,606,289 entitled Audio frequency power amplifiers with actively damped filter; U.S. Pat. No. 5,563,781 entitled Dual-mode power converter; U.S. Pat. No. 5,543,753 entitled Audio frequency power amplifiers with actively damped filter; U.S. Pat. No. 5,532,917 entitled Power factor corrected rectifying circuit; U.S. Pat. No. 5,510,753 entitled Composite bridge amplifier with output overload and thermal protection; U.S. Pat. No. 5,499,154 entitled Protective shut-down system for switch-mode power supply; U.S. Pat. No. 5,450,307 entitled Switching power source apparatus; U.S. Pat. No. 5,396,194 entitled Audio frequency power amplifiers; U.S. Pat. No. 5,345,164 entitled Power factor corrected DC power supply; U.S. Pat. No. 5,267,132 entitled High power switching power supply with high power factor; U.S. Pat. No. 5,233,509 entitled Switch-mode AC-to-DC converter; U.S. Pat. No. 5,224,029 entitled Power factor and harmonic correction circuit including ac startup circuit; U.S. Pat. No. 5,164,656 entitled Switching power supply with constant or sinusoidal input current; U.S. Pat. No. 5,155,430 entitled Switching power supply with constant or sinusoidal input current and with fixed or variable output voltage; U.S. Pat. No. 5,129,006 entitled Electronic audio signal amplifier and loudspeaker system; U.S. Pat. No. 5,075,634 entitled Composite bridge amplifier; U.S. Pat. No. 5,012,161 entitled Power factor correction circuit; U.S. Pat. No. 5,006,975 entitled Power factor correction circuit; U.S. Pat. No. 5,003,454 entitled Power supply with improved power factor correction; U.S. Pat. No. 4,980,812 entitled Uninterrupted power supply system having improved power factor correction circuit; U.S. Pat. No. 4,980,649 entitled Ultra efficient switching power amplifier; U.S. Pat. No. 4,961,044 entitled Power factor correction circuit for power supplies; U.S. Pat. No. 4,949,234 entitled High efficiency power factor correction circuit; U.S. Pat. No. 4,947,308 entitled High power switching power supply; U.S. Pat. No. 4,876,497 entitled Power factor corrector; U.S. Pat. No. 4,855,890 entitled Power factor correction circuit; U.S. Pat. No. 4,849,651 entitled Two-state, bilateral, single-pole, double-throw, half-bridge power-switching apparatus and power supply means for such electronic power switching apparatus; U.S. Pat. No. 4,831,508 entitled Power supply system having improved input power factor; U.S. Pat. No. 4,808,946 entitled Lightweight, high power audio amplifier and power supply; U.S. Pat. No. 4,683,529 entitled Switching power supply with automatic power factor correction; U.S. Pat. No. 4,677,366 entitled Unity power factor power supply; U.S. Pat. No. 4,672,300 entitled Direct current power supply using current amplitude modulation; U.S. Pat. No. 4,652,769 entitled Module power supply; U.S. Pat. No. 4,651,112 entitled Audio amplifier system; U.S. Pat. No. 4,555,672 entitled High-fidelity audio frequency amplifier; U.S. Pat. No. 4,464,634 entitled Audio power amplifier; U.S. Pat. No. 4,195,335 entitled Switching power supply having a wide range input capability; U.S. Pat. No. 3,487,289 entitled Multipurpose Power Converter Circuits; and U.S. Pat. No. 3,010,064 entitled Automatic Power Factor Corrector. Each of these patents is hereby incorporated by reference.
None of these patents disclose or suggest the advantages of the present invention specifically directed to the problems associated with an audio power amplifier. What is needed, then, is a Power Supply Regulation and Protection Circuit for an Audio Power Amplifier.
The present invention teaches an audio power amplifier including a power supply with a positive and negative rail output. The power supply is regulated by a feedback signal through a peak current mode control system using a voltage error amplifier. The invention utilizes a rail voltage detection circuit connected to a feedback generator and one of the rail voltages. The detection circuit is adapted to detect an overvoltage or undervoltage condition and control the feedback signal to the regulated switching power supply so that the power supply will adjust the rail voltage. Additional protection is provided through thermal stress detection and DC output detection to control the feedback signal and protect the power supply and the audio frequency amplifier.
The novel regulation scheme of the present invention allows partial voltage regulation of multiple outputs without the expense and complexity of a post regulator. Additional regulation is provided for overload and fault protection. The outputs of the power supply are regulated with a non-linear scheme that allows the output of the lowest magnitude, the loaded output, to dominate the control loop. This ensures full peak power operation during each half cycle. This control is allowed unless the higher rail, the unloaded rail, exceeds a maximum limit. If the maximum limit is exceeded, then the loop will regulate a combination of the output voltages to ensure that the maximum limit is not exceeded while still providing as much power as possible to the audio frequency amplifier. Either of these mechanisms is overridden in a non-linear fashion by additional circuitry, which detects the presence of DC on the amplifier output, which indicates a fault that would damage the load if allowed to continue. Finally, additional control input is provided in the form of thermal protection, where the voltage of all outputs is reduced as the thermal limits of power components are approached. This has the effect of lowering the power dissipation in the linear amplifier and the power supply, but does not shut down the amplifier completely. Equilibrium of the regulation scheme may be reached, where the amplifier is operated from the maximum supply voltage possible for given load, ambient, and program conditions.
The voltage regulation scheme of the present invention also protects against amplifier overload when driving impedances lower than recommended. The amplifier usually limits output current, but cannot reduce the supply voltage, leaving the amplifier in a high thermal stress condition. In the present invention, high power dissipation will cause the thermal feedback circuit to reduce the rail voltages, which in turn reduces the amplifier dissipation, protecting the amplifier without shutting it down. Thus, the dissipation is reduced by allowing the supply voltage to be reduced in response to an overload, thereby reducing the thermal stress in the output stage.
These and other advantages may be understood by reviewing the following detailed description of the preferred embodiment of the invention.